A laser diode emits light when a driving current thereof exceeds an oscillating threshold current (called simply threshold current). The intensity or power of emission thereof is proportional to a modulating current corresponding to the current exceeding the threshold current. In order to make fast the speed of response of an emitting operation of the laser diode, the threshold current or a neighboring current thereof is caused to flow at all times, and the modulating current is allowed to flow as a pulse current corresponding to a data signal in form superimposed on the bias current. As a result, an optical pulse can be generated.
The stable execution of optical communications needs to hold the intensity of light at its emission constant. At this time, the emission characteristic of the laser diode greatly depends on the temperature. Namely, the threshold current becomes great as the temperature rises. Further, the modulating current necessary to obtain predetermined emission power becomes also great as the temperature rises. The emission characteristic of the laser diode is deteriorated due to secular changes, and the threshold current becomes great as its using period becomes long. The modulating current necessary to obtain the predetermined emission power is also made great. Further, the characteristic change corresponding to the temperature and secular changes differs between the threshold current and the modulating current.
In order to cope with such a characteristic change, an auto power control technique has heretofore been adopted wherein the mean level of emission power of a laser diode is detected from a current flowing in a photodiode provided so as to be opposed to the laser diode and a bias current is increased by the amount equivalent to a reduction in the detected level, whereby fixed emission power can be obtained. Japanese Patent Application Laid-Open No. Hei 8-204268 is known as an example of a reference in which the present technique has been described.
In the above-described prior art, however, only the bias current is changed and no modulating current is controlled. Therefore, if the bias current exceeds the threshold current, then a quenching failure occurs. If the bias current is excessively smaller than the threshold current in reverse, then a quenching delay occurs. In a word, the prior art merely controls the sum of the bias current and the modulating current with respect to changes in threshold current of the laser diode and modulating current for obtaining fixed emission power of the laser diode due to a change in temperature and secular changes.
On the other hand, a technique for controlling a driving current of a laser diode while paying attention to both changes in threshold current corresponding to the temperature and modulating current for obtaining fixed emission power has been described in Japanese Patent Application Laid-Open No. Hei 6-61555. Namely, current ratio control data defining the ratio between the optimum bias current and modulating current for each operating temperature of the laser diode is prepared in a ROM or the like. The current ratio control data is read from the ROM in accordance with the result of detection of the operating temperatures of the laser diode, and reference is made to the driving current of the laser diode subjected to auto power control, whereby the bias current and modulating current are determined according to the current ratio control data with respect to the driving current.
However, the aforementioned prior art has no taken into consideration the deterioration in characteristic of the laser diode due to the secular changes. According to the discussions of the present inventors, it was revealed that if a distinction between whether an increase in driving current by the auto power control results from a change in ambient temperature and whether it results from deterioration in characteristic of the laser diode due to the secular changes would not be done, it was difficult to optimize both the bias current and the modulating current.
Further, the wavelength of output light also varies with the deterioration of the laser diode. This occurs because the laser diode is deteriorated and the driving current for obtaining the required optical output increases to thereby increase the temperature of an active layer of the laser diode and shift the output wavelength to the long-wave side. If the temperature of the active layer is lowered, then the wavelength of the optical output is shifted to the short-wave side. Such changes in wavelength cause a recognition error of a transmission signal in, for example, a system for performing wavelength-division multiplexing transmission.
An object of the present invention is to provide an optical transmission device capable of improving the reliability of light-based information transmission.
Another object of the present invention is to provide an optical transmission device which reduces a quenching failure and an emission delay to the minimum with respect to deterioration in characteristic of a laser diode due to a change in ambient temperature and secular changes to thereby make it possible to hold an optical output constant.
A further object of the present invention is to provide a method of reducing a quenching failure and an emission delay to the minimum with respect to deterioration in characteristic of a laser diode due to a change in ambient temperature and secular changes to thereby drive the laser diode.
A still further object of the present invention is to provide an optical transmission device capable of relaxing a change in the wavelength of an emission output.